HVAC systems can be used to regulate the environment within an enclosure. Typically, an air blower or circulating fan is used to pull air from the enclosure into the HVAC system through ducts and push the air back into the enclosure through additional ducts after conditioning the air (e.g., heating or cooling the air). For example, a gas furnace, such as a residential gas furnace, is used in a heating system to heat the air.
Residential gas furnaces are tested during manufacturing to insure compliance with government and industry standards. For example, residential gas furnaces must pass a 100 day heat exchanger corrosion test per ANSI 21.47 requirements. This corrosion test is a cyclical test of four minutes of the burner on and eight minutes of the burner off. The corrosion test must be conducted with the circulating fan of the heating system continuously energized. Modulating or two-stage gas furnaces must pass the corrosion test at both low and high firing rates. At the low-fire rate, heat exchanger temperatures are significantly lower compared to the high firing rate. As such, it is more difficult to pass the corrosion test at the low-fire rate compared to the high-fire rate. Accordingly, some manufacturers have used expensive stainless steel materials, complicated internal flue baffling, increased the minimum firing rate, or reduced the overall furnace efficiency to pass the corrosion test at the low-fire rate.